What is the history of the 'Passover'
The Passover as it is explained in the book of Exodus in chapters 11 and 12 is an event that had to do with freeing of the Israelites from Pharaoh. Pharaoh would not let the people go as demanded by God through Moses. God had put several plagues against Pharaoh and the final one was that all first born males were killed, both young and old, human and animal.
“For I will pass through the land of Egypt on that night and will strike the firstborn in the Land of Egypt, both man and beast; and against all the gods of Egypt I will execute judgment: I am the Lord.” Ex. 12-12
Then God instructed Moses to tell the people of Israel to place the blood of a lamb over the door of their houses. This blood was a sign for the wrath of God to 'pass over' that house so the people of Israel would be spared.
"Now the blood shall be a sign for you on your houses where you are. And when I see the blood, I will pass over you; and the plague shall not be on you to destroy you when I strike the land of Egypt.” Ex. 12-13